The successful operation of any satellite communication system requires a capability to accurately determine the orbits of all satellites involved--whether they are in geosynchronous or low earth orbits. Also of importance is a growing interest in accurate, on-board navigation capabilities for low-earth orbiting (LEO) satellites. In particular, future requirements for earth observation missions call for LEO spacecraft orbit determination (OD) accuracies approaching the 10 m (1.sigma.) level. Supporting this capability with Tracking and Data Relay Satellite System (TDRSS) implies orbit determination (OD) accuracies for TDRSS satellites in the 25 m (1.sigma.) range.
Of the available techniques for orbit determination, pseudo-noise (PN) and tone-ranging-based multilateration potentially permits near real-time operation. PN ranging is a key feature of the Global positioning System (GPS) and currently is extensively used by the Tracking and Data Relay Satellite System. Tone ranging is another well established technique, but its utility is typically limited by narrow antenna beam width requirements on space-to-ground transmissions due to flux density concerns. Interferometry and, in particular, Long Baseline Interferometry (LBI) utilizing stellar radiation sources, is taking on increasing significance for the precise determination of ground-station locations. Ongoing studies and experiments are being conducted to assess the feasibility of applying interferometry to TDRS orbit determination.
For orbit determination processes which rely on propagation delay and delay-difference measurements, as with PN/multilateration and LBI, availability of sufficient signal bandwidth is the key to providing a fine resolution time measurement capability (e.g., to the nanosecond level or less). Unfortunately, even if sufficient contiguous bandwidth is available (e.g., 50-100 MHz;, hardware and processing limitations may preclude a system from taking full advantage of the bandwidth resource. In addition, external error sources, such as ionospheric delay, may introduce bias errors that dramatically degrade range measurement accuracy, and are not easily calibrated out.
in interferometry, where the requirement for high rate data transfer from the dispersed observation sites to a central processor severely limits contiguous bandwidth usage, a technique termed "Bandwidth Synthesis" (BWS) has been developed. This involves the processing of energy in several narrow, disjoint frequency bands in a manner that effectively leads to a time resolution capability corresponding to the entire bandwidth spanned by the disjoint bands. Accordingly, a wide bandwidth (e.g., &gt;50 MHz) can be synthesized from several distinct narrow bandwidth components (e.g., 2 MHz apiece).
The object of the present invention is to provide an improved ranging and timing system for tracking and data relay satellite system in which the bandwidth synthesis technique is incorporated in pseudo-noise ranging to produce a system having unique properties and capabilities not presently available via other techniques. According to the invention, a precise ranging and timing system incorporating the pseudo-noise bandwidth synthesis technique provides precise orbit determination for geosynchronous and low earth orbit satellites. The system is predicated on a novel signal structure which is comprised of several disjoint, narrow-band spectral components spread over a wide bandwidth. The number of spectral components, their individual bandwidths, and their specific spectral locations over the end-to-end spread bandwidth determine the ultimate capabilities and performance achievable. The broad beam transmission of the precise ranging and timing system signal, via geosynchronous satellite, provides the system capability suitable equipped users for multiplicity of purposes including precise orbit determination, navigation of low-earth orbiting satellites through signals transmitted through geosynchronous satellites and precise time transfer.
Accordingly, the use of pseudo-noise bandwidth synthesis (PNBWS) according to the invention, overcomes a variety of operational concerns and provides enhanced performance benefits. In particular, PNBWS does not demand the use of hydrogen masers that may be required in ultra-fine resolution interferometric applications. Also, data transfer rate requirements should be much lower for PNBWS since intererometry involves the transfer of unprocessed data, while PNBWS only requires that low rate data (e.g., &lt;1 kbps), reflecting derived range information, be transferred among sites. Utilization of the known coherent, PN code signal structure may further permit simplified means for ambiguity resolution, self-contained ionospheric delay calibration, and self-contained network clock synchronization. Finally, PNBWS-related enhancements lead to significant range measurement accuracy improvements which, in turn, may ease network geometry considerations. In particular, in the TDRSS context, an all Continental United State (for example) based network of PNBWS ground terminals may meet or exceed TDRSS orbit determination accuracies currently achievable via the Bilateration Ranging Transponder System (BRTS), which employs non-continental united state terminals.
The key features of novel PNBWS tracking technique as disclosed herein:
1) time-of-arrival measurement accuracies to within 0.1 NS RMS potentially achievable, PA1 2) combines benefits of PN ranging, tone ranging, wideband interferometry, while overcoming limitations of prior systems, PA1 3) provides self-contained capability for accurate calibration of key error sources (ionosphere, group delay, clock biases), PA1 4) enhances currently proposed advanced TDRSS (ATDRSS) navigation beacon, PA1 5) simultaneously applicable to: PA1 1) improved ephemeris tracking accuracy, .ltoreq.25 M RMS compares to 50-100 M for existing bilateration ranging transponder system (BRTS) PA1 1) improved TDRS ephemeris accuracy, PA1 2) reduced sensitivity to dynamic modeling errors (gravitational, drag) PA1 3) precise pseudo-ranging via PNBWS processing
a) TDRS orbit determination PA2 b) user spacecraft on-board navigation PA2 c) time and frequency transfer among widely separated space and/or ground-based terminals. PA2 2) no ground stations outside Continental United States (CONUS) required, PA2 3) accurate calibration of key error sources, PA2 4) no scheduled forward link resources required, PA2 5) minimal MA return link resources required.
Key benefits of the invention to TDRS orbit determination:
Key benefits of the invention to TDRSS user navigation: Enhanced Beacon Navigation performance Via:
The uniqueness of the pseudo-noise bandwidth synthesis concept principally arises from its novel signal structure, which, as noted above, comprises several disjoint, narrowband spectral components spread over a wide bandwidth. The number of spectral components, their individual bandwidths, and their specific spectral capabilities and performance achievable. While the bandwidth synthesis concept was developed for interferometric signalling purposes, according to the present invention, the added intelligence and a priori information embedded in the pseudo-noise bandwidth synthesis signal not only enhances the corresponding interferometric capability, but simultaneously combines benefits and overcomes limitations of both pseudo-noise and tone ranging. For example, the pseudo-noise bandwidth synthesis according to the invention provides a much greater bandwidth than conventional pseudo-noise systems, at no increase in complexity. In addition, the wideband time resolution achievable via the tone ranging is also achievable by way of the present invention but the present invention further satisfies flux density constraints and may afford enhanced ambiguity resolution relative to conventional tone ranging; the latter is based on the simultaneous PN code presence, with additional housekeeping data superimposed.